The invention relates to an injection system, in particular in apparatuses for injection-moulding of plastics.
In particular, the invention relates to an injection system according to the preamble of the first claim.
An injection system of this type is already known, for example from the patent publications WO 01/78962, WO 03/064135 and WO 2008/131517.
Known systems for injecting plastics in the field of apparatuses for moulding are improvable in various aspects.
In the first place it is desirable to reduce the undesired effects caused by heat expansion, which is mainly caused by the heat in the runner manifold. These undesired effects may comprise dimensional variations to parts of the system such as to cause, for example, leaks of the operating fluid (for example air) of the valve stem actuator, imprecisions in the operation of the valve of the nozzle, incorrect balancing of the various pistons (mounted in series on the plates or slabs of the injection system), imbalance in the loads supported by the various elements (nozzles, cylinders, bushes, etc) mounted serially on the plates of the system, etc.
It is further desirable to maintain a compressing load that is not excessive but is nevertheless able to provide the correct clamping force for clamping the hot runner manifold between the manifold plate and the back plate, to prevent the escape of the injected plastics, whatever the operating condition (in particular at any temperature of the hot runner), and further to ensure correct preloading of the elements interposed between the hot runner manifold and the back plate (for example the piston cylinders of the actuators) and of the elements interposed between the hot runner manifold and the manifold plate (for example the injection nozzles).
Another improvable aspect is to increase the duration of correct operation of the valve stem actuator. It is noted that a drawback in this respect of known injection systems consists in that the seal means (seal) of the valve actuator can easily get damaged, for example owing to the sliding on a hot wall and/or the deformation (stretching) suffered by the seal whilst it is inserted into the seat thereof.